This invention relates to a control valve for a vacuum brake booster. The control valve has a plunger which is urged to a rest position adjacent an annular valve seat by a static pressure differential created across the plunger and a poppet member. A resiliently positioned sleeve concentric to said plunger delays the movement of the plunger by the static pressure differential until after an operational pressure differential in the vacuum brake booster has been terminated.
In my earlier U.S. Pat. No. 4,487,105, I disclosed a control valve which is moved from a fully released position to a rest position by a static pressure differential to reduce the distance required for an input member to move a control valve in response to an imput force in effecting a brake application. Under some operating conditions, it has been found that the movement of the control valve by this static pressure differential occurs prior to the complete termination of the operational pressure differential developed during a prior brake application. Under such circumstances, the termination of the operational pressure differential is delayed and as a result the retraction of the brake lining or pad with respect to a corresponding rotor or drum is also delayed.
As disclosed in both U.S. Pat. Nos. 4,487,105 and 4,562,696, a key member which engages the wall of a housing can be used to establish the relationship between the annular seat and plunger. However, if an operational pressure differential has not been terminated prior to the positioning of the plunger by the key member, a similar delay can also occur in the termination of the operational pressure differential since a limited flow path is established through which air can be evacuated from the rear chamber by way of the control valve to the front chamber.
The control valve structure disclosed in U.S. Pat. No. 4,358,990 discloses the relationship between a valve seat and poppet member which changes as a function of the retraction forces would not appear to solve the early movement of the plunger to a position adjacent the valve seat as the wall is returned to the rest position since the poppet member position is tied to the wall position.